Method:
Melt butter in a medium saucepan, over medium heat. Whisk in flour until it absorbs butter and turns into a roux; being careful not to brown flour. Whisk in mustard, hot sauce, salt and pepper. Stir in beer to integrate with the roux. Stir in cream and egg yolk. Continue whisking until incorporated. Stir in cheese; in batches, whisking gently until it melts and mixture is smooth and creamy. Remove away from heat. For a twist: you can add to the mixture, at this point, either shredded pieces of pastrami or thin slices of sausage, pitted black olives (chopped), chives, parsley, diced tomato; whichever addition you desire, or plain. Place bread slices on a baking tray. Pour mixture equally over each slice of toast. Place under the grill for one minute. Remove and serve immediately.